Because of an increasing interest in environmental issues, aliphatic polyesters having biodegradability have been intended to apply to fibers, molded articles, films, sheets, and the like, as resins for further avoiding environmental burden. For example, since polybutylene succinate and/or polybutylene adipate having biodegradability have mechanical properties resemble to those of polyethylene, they have been developed as alternative polymers for polyethylene.
As an economically advantageous process for producing an polyester, there is known and adopted since a long time ago a process for producing a polyester having a high degree of polymerization wherein an ester oligomer is produced by a direct esterification reaction of a dicarboxylic acid with a diol or an ester exchange reaction of an alkyl ester of a dicarboxylic acid with a diol in the presence of a catalyst and then the polyester having a high degree of polymerization is produced by subjecting the oligomer to an ester exchange reaction under heating and under reduced pressure with removing the diol formed by distillation.
However, since thermal stability of an aliphatic polyester is generally low and hence a decrease of the molecular weight due to thermal degradation is caused during the polymerization reaction, it is impossible to obtain a polyester having a high degree of polymerization which has a practically sufficient strength by a conventional process for producing the polyester. It is proposed that the concentration of the polymer terminal (hydroxyl group or carboxyl group), particularly the remaining carboxyl group remarkably affects adversely thermal stability of polymer (e.g., cf. Patent Document 1). Based on such a background, various kinds of contrivance are applied to the production process.
For example, there are proposed processes for enhancing melt viscosity of the polymer through extension of the polymer chain length by carrying out melt polymerization using a titanium compound or a zirconium compound as a catalyst with adding a diisocyanate (e.g., cf. Patent Document 2) or a diphenyl carbonate (e.g., cf. Patent Document 3). Since these processes of adding the chain extenders can easily increase the molecular weight of the polyester, the processes are apparently considered to be effective production processes for aliphatic polyesters but there are problems that the reaction is carried out in two steps and thus is complicated and also, with regard to the resulting polyester, in addition to a slight decrease of crystallinity and melting point thereof, biodegradability of the resulting polyester tends to decrease owing to the urethane bond contained in the molecule.
Moreover, there is disclosed a process of converting the structure of the polyester into a crosslinked structure by adding a trifunctional oxycarboxylic acid in an amount of 0.5 to 5 mol % or a tetrafunctional oxycarboxylic acid in an amount of 0.1 to 3 mol % as a branching agent (e.g., cf. Patent Document 4). However, the polyester wherein melt viscosity is enhanced by introducing a large amount of the trifunctional or tetrafunctional oxycarboxylic acid as above exhibits a tendency to increase the concentration of the polymer terminal (hydroxyl group or carboxyl group) which is a cause for decreasing thermal stability and also has insufficient practical physical properties. Therefore, in most cases, there is applied contrivance that the terminal number in the polymer is reduced and also the molecular weight of the polymer is enhanced by adding a diisocyanate at the late stage of the polymerization (e.g., cf. Patent Document 5).
Furthermore, there is also proposed an aliphatic polyester wherein elasticity is enhanced by incorporating a dibasic acid having a hydroxyl group into an aliphatic polyester in an amount of 0.05 to 5% by weight (e.g., cf. Patent Document 6). Since the actually produced polyester in the Example has a content of the dibasic acid of so much as 1 to 2 mol %, thermal stability thereof tends to decrease and also, as mentioned above, a chain extension is conducted by further adding a diisocyanate.
Additionally, it is proposed that, as a polyester carbonate, by reducing the content of specific dicarboxylic acid impurities including malic acid as a trifunctional oxycarboxylic acid in the starting dicarboxylic acids to 0.4% by weight or less, reproducibility of the amount of gel formation, thermal stability, color tone, and moldability at the molding of the polyester carbonate is improved (e.g., cf. Patent Document 7). In this case, the lesser total content of dicarboxylic acid impurities (malic acid, maleic acid, and fumaric acid) is considered to be preferable but a polyester having a high degree of polymerization is also not obtained in this case, so that a carbonate compound which is an chain extender is still added.
On the other hand, there are proposed several processes for producing a high-molecular-weight one without using a chain extender such as an isocyanate or a carbonate.
For example, in order to enhance the rate of the polymerization reaction, there are disclosed a process of carrying out dehydrative condensation with azeotropic removal of water formed during the reaction and an organic solvent in the solvent using a tin compound as a catalyst (e.g., cf. Patent Document 8) and a process of carrying out a polycondensation reaction under vary high vacuum of 0.005 to 0.1 mmHg (e.g., cf. Patent Document 9). However, since these production processes, especially the latter process produce a polyester substantially having hydroxyl group terminals, they are expected as processes for producing polyesters excellent in thermal stability from the aforementioned viewpoints but have disadvantages that not only the production steps are complicated but also extremely large investment in facilities is necessary. Moreover, since this process takes a long period of time for production of the polyester having a high degree of polymerization, there are fears of thermal degradation and coloration of the polymer during the production.
Furthermore, as the other process, there is proposed a catalyst system of combining a proton-releasing phosphorus compound such as an organic phosphinic acid and a hydrogen phosphate salt and a polymerization catalyst (e.g., cf. Patent Document 10). These proton-releasing acidic compounds not only generate by-products such as tetrahydrofuran from starting butanediol (Encyclopaedia Chimica, vol. 7, p. 850, Kyoritsu Shuppan (1962)) but also possibly deteriorate thermal stability and hydrolysis resistance of the polyester by increasing acid concentration in the final product.
As a method for overcoming such various problems, the present applicant has proposed that a polyester having a high degree of polymerization can be easily produced by adding a bifunctional oxycarboxylic acid such as lactic acid to the polymerization components to form a ternary system (1,4-butylene glycol, succinic acid, and lactic acid) or a quaternary system (1,4-butylene glycol, succinic acid, adipic acid, and lactic acid) and using a Ge-based catalyst as a catalyst (e.g., cf. Patent Document 11). Moreover, for the purpose of further enhancing melt viscosity, a process of adding a trifunctional oxycarboxylic acid to the above polymerization systems has been proposed (e.g., cf. Patent Document 12). However, in the process of adding lactic acid to these polymerization system, since lactide which is a cyclic dimer of lactic acid is apt to be generated at the heating, during the polymerization reaction, not only problems such as blockage of reaction tubes are sometimes induced but also a polyester containing a lactic acid component has a slight odor of lactic acid or thermal degradation and coloration are caused owing to generation of lactide or the like under a high temperature condition in some cases.
Furthermore, such an aliphatic polyester exhibiting biodegradability generally has a characteristic that it is apt to undergo a hydrolysis reaction and hence there still remains a problem of improving durability of mechanical properties such as tensile properties for relatively long-term storage and use. As a method for improving hydrolysis resistance, there is proposed a method of mixing an aliphatic polyester with a carbodiimide compound (e.g., cf. Patent Document 13). However, the effect is not sufficient, for example, the tensile elongation percentage at break decreases to less than 50% of the initial value after four weeks of test, and thus there is practically a serious problem.
[Patent Document 1]
JP-A-7-53700
[Patent Document 2]
JP-A-4-189822
[Patent Document 3]
JP-A-8-301999
[Patent Document 4]
JP-A-5-170885
[Patent Document 5]
JP-A-5-178956
[Patent Document 6]
JP-A-5-271377
[Patent Document 7]
JP-A-11-60709
[Patent Document 8]
JP-A-9-77862
[Patent Document 9]
JP-A-5-310898
[Patent Document 10]
JP-A-2002-187943
[Patent Document 11]
JP-A-8-239461
[Patent Document 12]
JP-A-8-259679
[Patent Document 13]
JP-A-11-80522